Although Boss does refer to the supplied adapter as an "AC Adapter" (probably because it gets power from an AC mains wall outlet) the PSA series power adapter that comes with the ES-8 outputs 9VDC. The rear panel of the ES-8 says "DC In" above the power jack.

The ES-8 takes 9VDC and draws just under 400mA. It can be powered from the ISO-5, Digital, 4x4, or Mondo Pedal Power models.

With a Pedal Power 2+ you would combine outputs 5 and 6, with their corresponding DIP Switches set to Normal, using a Current Doubler Adapter (the one with the Green end) together with one of the included all black standard barrel cables.
Alternatively, you could use the power adapter that came with the ES-8 and connect it to the Courtesy Outlet on the back of the Pedal Power 2+.